Electrical amplifier circuits



June 6, 1933. F. H. DRAKE 1,912,891

ELECTRICAL AMPLIFIER CIRCUITS Filed Oct. 51. 1928 2 Sheets-Sheet l June 6, 1933.

F. H. DRAKE ELECTRICAL AMPLIFIER CIRCUITS Filed Oct. 31, 1928 2 Sheets-Sheet 2 Patented June 6, 1933 UNITED STATES PATENT OFFICE FREDERICK H. DRAKE, OF

BOONTON, NEW JERSEY, ASSIGNOR, BY MESNE ASSIGN- MENTS, '10 RADIO CORPORATION OF AMERICA, OF NEW YORK, N. Y., A CORPORA- TION OF DELAWARE ELECTRICAL AMPLIFIER CIRCUITS Application filed October 31, 1928. Serial No. 316,207.

This invention relates to electrical circuits of the character of electrical amplifier circuits and the like and especially to such circuits of the type employing electron or vacuum tubes. Such tubes may include a closed vessel, an electron emittingelement or cathode, an electron-receiving anode or plate, and one or more elements or control means which ordinarily take the form of grids or the like. Such tubes are commonly known as audions, triodes, tetrodes, and so forth. My invention relates more particularly to a neutralized electrical amplifier circuit of the foregoing type, and provides a method of and means for reducing or suppressing retroactive or regenerative currents due to coupling between output and input circuits of such an electrical amplifier. The retroaction or regeneration which it is desired to reduce or suppress may be due to capacitive coupling between these circuits, and especially to capacity between the anode and control element of the tube, this capacity being augmented by stray capacities between external connections and circuits associated with these elements, and my invention provides a novel and improved method of and means for neutralizing this capacity coupling. My invention relates also to the design, proportions, and circuit constants of an electrical amplifier circuit including a radio frequency transformer, and to such a circuit having certain electrical relations between elements thereof, these relations being of such a nature as to tend to reduce undesired or disturbing regenerative or feed-back action through the capacitive coupling above referred to. By means of my invention this regenerative or feed-back current may be reduced to smaller values than are ordinarily obtainable with the simple balance conditions and adjustments commonly employed. My invention relates also to a three-coil or three-winding transformer for use in conjunction with a tube of the above described type in an electrical amplifier circuit, and to such a transformer having a primary wmding or input coil adapted for connection between the anode and cathode of a tube,

a secondary winding or output coil which may be tuned by avariable condenser if desired, and a third winding or neutralizing coil electromagnetically coupled to the primary winding and connected through neutralizing capacitive impedance to the control element of the tube, the secondary winding being coupled electromagnet-ically to both the primary winding and the neutralizing winding, as is characteristic of the so-called neutrodyne circuit. In such a three-coil transformer and electrical circuit associated therewith the electromagnetic coupling between the several windings may assume a certain range of values, as will be hereinafter described; and within this range the coupling between the primary winding and the neutralizlng winding may be less than substantially unity coupling, but, within the scope of my invention, may also, in certain as Wlll be substantially equal to unity, appear from the description which follows:

I am aware that electrical circuits employing transformers of the three coil type above described have hitherto been extensively employed in connection with so-called neutrodyne radio frequency amplifiers. My invention, however, relates to such a three coil transformer, and to an electrical circuit associated therewith, proportioned and con structed in a novel manner, and having a certain particular, novel relation or relations, among the electrical constants and. circuit elements thereof, as is hereafter described and explained.

In the accompanying drawings Figure 1 is a diagrammatical representation of an electrical amplifier circuit including a tube of the three-element type and a three coil transformer associated therewith. Figure 2 illuscases,

trates a similar arrangement employing a tube of the four-electrode or double-grid type. Figures and 4t illust ate diagrammatically examples of certain forms of transformer with which the electri-al relations according to my invention may be sibstantially realized.

Referring; now particularly to Figure 1, the tube T is provided vit-h anode P, control element or grid G. and cathode F. An input circuit of any desired type is illustrated dia grannnatically as being connected between the grid and cathode, and has an impedance represented by Zg. The A, B and C batteries, or iqnivalent socket-power or other devices. or alternatingcurrent filament supply means, may be arranged in any convenient manner. The czpacity Cm. illustrated in dotted lines. represents the disturbinginter-electrode capacity which causes teed-back from the plate or output circuit to the grid or input circuit. Cm includes the inherent grid-plate c: picity, together with stray external capacities which exist effectively between the grid and plate terminals of the tube and operate to augment the effective tube capacity. A. three coil transformer L. L L;; associated with the tube, the primary winding I and B-pi' tcntial source being connected in series between anode P and cathode I and the secondary winding" L being coupled therewith and bein; tuned by variable condenser C: to form a resonant outputcircuit having output terminals 3 and l. The neutralizing coil or winding. L is electriimagnetically coupled with the primary winding I and is connected ctl'ectively at one terminal to the cathode, and at the other ter ninal to the grid through the neutralizing capacity to form a neutralizing circuit. The coil L and L; are either wound in opposite directions, and connected as in Figure 1, or are wound in the same direction. with reversed connections, so that the mutual inductance between them s eil'ectivel negative. The mutual induc- .lti between the coils I1 and L is desigttcd as M 3: the mutual inductance between the coils L and L as M and the mutual inductance between the coils L and L as M 1 The output vol age of the amplifier is design: ted as IQ. which is the voltage available across the tuning condenser C The input voltage across the input impedance Zg between grid and cathode is designated as E The alternatingcurrent plate current, 2], flowing through the. coil I1 produces across this coil a voltage which feeds back through the capacity Cm a disturbing potential onto the grid or input terminal. G, of the tube. By virtue of the neutralizing winding L elm-tromagz'nctically coupled to coil L and izing' capacity (L. connected in series therewith. a voltage may be ted back to the grid in such a phase and amplitude as to oft-set to a conside "able extent the effect The condit on of equation is imposed in order that the balance shall be independent of frequency. and has the physical meaning that the coils L and I are coupled with unity coupling. Equation gives the required 'alue oi the neutra izim: capacity (I; in terms ot' the capacity C and the ratio of the primary inductance I1, to the mutual inductance M In general. I have determined both tl :oreticall and by experiment. that the coni dition given in equation (1) above of far less practical importance than the condition given in equation. (E2). A considerable departure from the condition of equation (1) still permits an exact balance of the network tor a single frequency. and a good approximation to balance over a considerable range of frequencies. If equation (1) is not satistied. then exact balance is obtained for the frequency when It 7 1, equation reduces to equation (2) above. Furthermore a con'iputation of the term C... I L 0 shows that in many practical cases this term negligible compared with M so that a balance sensibly independcut of frequency may be obtained without the use of unity coupling.

Equation above will hereinal'tmbe termed the primary balance condition. and equation (1) above will be termed the unity coupling condition.

Turning now to the case of the three-coil transformer as shown in Figure 1 when the circuit IJ C is tuned for maximum E the current '1': flowing in L C: will in general be large compared with the currents flowing through L and L The voltages produced in coils I1, and L by i are determined by the mutual inductances M 1 and M In order that these voltages may not affect the grid potential, the neutralizing capacity C must have the value:

E nation (3) is herein termed the secondary balance condition. This is substantially a frequency-free balance for any degree of coupling in the transformers, and is the balance usually obtained in practical radio frequency amplifiers of the Neutrodyne type by the common method of impressing a signal on the grid and filament terminals of the cold tube and adjusting C for minimum vo tage across C With the foregoing discussion by way of explanation, I may now state that I have found, as result of both theoretical. and experimei'ital' investigation, that the condition for null effect upon the grid or input terminal of the tube for arbitrary currents i. and i in the complete three-coil. system illustrated in Figure 1, is the simultaneous fulfillment of both be primary and secondary balance conditions stated above: that is, the simultaneous fulfillment of the following conditions The feedback or retroaction produced by a departure of G from the value prescribed by equation is much less serious than that produced by asimilar departure from the condition of equation because 1'. is usually much greater than 'i, and it has already been StfifQfldllOVQ that the feedback produced by d eparture from the condition of equation (,1) is small compared with that produced by a similar departure from equation (2'). \Ve have therefore in practice a substantially frequency-free multaneous balance condition giving negligible feedback to the grid or input terminal if:

Eqiiiations 2 and 3 may be simultaneously satisfied if, and only if:

L lie. 4) fo Equation (4.) above is the relation between the various self and mutual inductances shown in Fi lure 1 which mu st obtain in order that a single adjustment of the neutralizing capacity G will produce simultaneous primary and secondary balance, i. e., balance for Y arbitrary values of 2' and i and this condiance has been obtained in the familiar way.

In some instances the voltage E may be considerably degenerated because of primary unbalance, even though exact secondary bal-* ance obtains. According to my invention I have determined relations which, if satisfied, will produce a simultaneous balance for both primary and secondary currents, so that, with proper shielding and by-passing as is now well known in the art, a true one-Way amplifier stage may be obtained with this type of neutralized circuit, without the necessity of en'iploying any particular degree of coupling, such, for example, as unity cou-' pling which has been found to have many disadvantages. Hitherto it has always been necessary in this type of circuit, so far as I am aware, to employ substantially unity coupling between primary and neutralizing coils, with lumped coils for these windings, in addition. to shielding and by-passing, in order to obtain a true one-Way amplifier stage.

By the use of my invention it is possible, also, to produce, as an alternative to complete simultaneous balance as described above, deliberate controlled departures from primary balance of such a nature as to modify the curve representing amplification as a function of wavelength, while maintaining exact secondarybalance. For example the coils L L and L may be so relatively proportioned that a degenerative departure from the primary balance condition is obtained, reresulting in a greater depression of the short wave gain than of the long wave gain. For condenser-tuned systems this results in a more uniform amplification characteristic.

The fulfillment of the conditions expressed in equation (4) above, I have found, imposes certain requirements upon the geometry and relative arrangement of the windings of the transformer. For example, in one particular circuit which I have successfully employed, the circuit constants and geometry of the windings were as follows:

Figure 3 illustrates in a diagrammatical manner, in longitudinal cross-section, a. transformer having the constants and geometry described above. In this arrangement the position of the primary winding L is fixed relative to that of the secondary winding L Adjustment for simultaneous balance is made by sliding the neutralizing coil L in the direction of the arrows. If simultaneous balance is obtained with the coils as shown, regenerative unbalance may be obtained by sliding the coil L from this balance position toward the high potential or grid end of the coil L and similarly a degenerative primary unbalance is obtained by moving L from its balance position toward the filament end of the winding L. It will be observed that when the coil L is moved as described above, the mutual inductance M varies at a different rate from the ratio g thus mak- 32 ing the adjustments effective to accomplish the simultaneous balance condition, or degree of unbalance, which is desired.

It should be understood that my invention is not limited to the exact fulfillment of the n'lathematical relations set forth herein, since in certain physical embodiments and structures involving my invention such exact fulfillment may be difficult or impossible to accomplish. A substantial or approximate fulfillment of these mathematical conditions in a physical structure is therefore definitely within the scope of my invention. Such a. substantial fulfillment of the foregoing mathematical conditions may be accomplished by the structure of Figure 3, or by other suitable arrangements, of which a further example is that shown in Figure 4 in which coil L is a single layer solenoid of conventional design, while coils L and L are shorter coils which may be independentlv rotated about an axis K in order to obtain the magnetic couplings with L and with each other which are required in order to satisfy the above simultaneous balance condition. It is clear that many variations of this particular construction may be alternately employed, and Figures 3 and 4 are intended to be illustrative only. lVith such constructions as those of Figures 3 and 4, I have determined experimentally that complete balance may be obtained in actual practice. and that by changing the relative position of the coils a regenerative or degenerative unbalance may be produced as desired, as described above.

Figure 2 illustrates an electrical network including a three coil transformer of the type above referred to and a tube of the fourelectrode or double-grid type. My invention is of course equally applicable to networks such as that herein illustrated, and extends also to arrangements in which the neutralizing winding L is connected to the grid Gr in such a manner that the capacity between the grids Gr and G: constitutes the neutralizing capacity C In such an arrangement the inherent capacity between the elements of the tube may be augmented by external capacities if desired. My invention applies also to electrical networks including double-grid tubes of the type de signed for use according to the method which comprises employing the grid, G as the control element, and applying to the grid G a positive bias.

It should be understood that the circuit diagrams of Figures 1 and 2 are merely illustrative of my invention. These circuits are, of course, applicable to multistage amplifiers as well as to single stages, as is well understood in this art. Moreover, the A, B and C batteries may, of course, be replaced by suitable socket-power devices, the C-batteries may be replaced by grid-biasing resistances, or other suitable means and the oathodes may be of the alternating current or separate heater types. The particular transformer constructions shown in Figures 3 and i are also given by way of example only. Other modifications and variations of the specific circuits and transformer structures herein described fall within the scope of my invention.

I claim:

1. An electrical amplifier circuit comprising. in combination, an electron tube having at least an anode, a cathode, and a control element; an input circuit associated with said control element, an output circuit including a threecoil transformer having a primary winding connected between anode and cathode, a neutralizing winding coupled to said prin'iary winding with less than unity coupling and so as to have effectively negative mutual inductance therewith and connected through a neutralizing capacity to said control element, and a tuned secondary winding coupled to said primary and neutralizing windings, the ratio of the mutual inductance between said tuned secondary winding and said primary winding to the effective mutual inductance between said tuned secondary winding and said neutralizing winding being substantially equal to the ratio of the selfinductance of said primary winding to the mutual inductance between said primary winding and said neutralizing winding.

2. An electri ral amplifier circuit comprising in combination an electron discharge device having at least an anode, a cathode and a control element; an input circuit associated with said control element; an output circuitii'icluding a transformer having a primary winding connected between said anode and said cathode, a secondary winding coupled to said primary winding, and a neutralizing winding with less than uni-decoupling and coupled to said primary winding so as to have eflectively negative mutual inductance therewith; and neutralizing capacity connected between said control element and said neutralizing winding, said transformer being so proportioned and constructed as to produce simultaneous balance of said electrical amplifier circuit for both primary and secondary currents and means supporting two of said windings for movement relative to each other and to the third of said windings, whereby the mutual inductances may be adjusted to such relative values that said simultaneous balance of said electrical amplifier circuit may be effected.

In testimony whereof, I aflix my signature.

FREDERICK H. DRAKE. 

